Friend first missing from motor race (4)
I believe the answer is:
ally
'friend' is the definition.
(an ally is a friend or supporter)
'first missing from motor race' is the wordplay.
'first missing from' means to remove the first letter.
'motor race' becomes 'rally'.
'rally' with its initial letter taken off is 'ALLY'.
(Other definitions for ally that I've seen before include "One on one's side" , "Team up" , "often helpful when cluing adverbs" , "One co-operating with another" , "He fights on your side" .)